Rare earth element magnet

ABSTRACT

A rare earth element magnet comprising molded magnetic powder containing at least one rare earth element, wherein a Fe rich phase covering a part or entire of the surface of particles of the magnetic powder and having a Fe atomic percentage larger than that of the magnetic powder, and an inorganic binder bonding the particles covered with the Fe rich phase.

CLAIM OF PRIORITY

The present application claims priority from Japanese application Serial No. 2007-122945, filed on May 8, 2007, the content of which is hereby incorporated by reference into this application.

FIELD OF THE INVENTION

The present invention relates to a rare earth element magnet and a method of manufacturing the same. And more particularly, to a magnet that can keep high magnetic characteristics under conditions of high temperature and high pressure and a method of manufacturing the same.

The magnet according to the present invention is suitable for use in permanent magnets. The magnet according to the present invention can be applied to fields wherein ordinary magnets are used, for example electric rotating machines.

RELATED ART

Compacted magnets with mechanical strength, which are produced by bonding magnetic powder with SiO₂ have higher residual magnetic density at room temperature due to high density and better temperature dependency because of an inorganic bonding agent, compared with magnets with mechanical strength by bonding with an organic material such as epoxyresins. Examples of the permanent magnets bonded with an inorganic material are disclosed in Japanese patent laid-open 10-321427, wherein rare earth element magnetic powder is coated with SiO₂ particles and the coated powder is compacted, followed by heat treatment at 200° C. to evaporate a solvent to bond the particles of the powder so that a permanent magnet featured by high electric resistance is obtained. On the other hand, Japanese patent laid-open 8-115809 discloses that SiO₂ is impregnated into oxide magnetic powder having been compacted to thereby produce a permanent magnet with excellent heat resistance and high mechanical strength.

Patent document 1: Japanese patent laid-open 10-321427

Patent document 2: Japanese patent laid-open 8-115809

SUMMARY OF THE INVENTION

Although a mechanical strength may be obtained by bonding the powder only with the inorganic material, i.e. SiO₂, oxidation and corrosion of the magnetic powder whose main phase is Nd₂Fe₁₄B may occur under severe environment such as high temperature and high pressure so that magnetic characteristics become worse. Further, since reliability of newly formed active surfaces that are formed by cracks, the magnetic characteristics may become worse. It is an object of the present invention to suppress the degradation of the magnetic characteristics. The means for solving the above problem is not disclosed in the patent documents.

In order to solve the problem, it is useful to form a suppressing phase having high corrosion resistance and thermal demagnetization suppressing effect at boundaries of magnetic particles whose main phase is Nd₂Fe₁₄B. A thickness of the suppressing phase should preferably be as thin as possible to separate the SiO₂ and the main phase so that a volumetric rate of the main phase is not lowered. This suppressing phase comprises Fe₁₇Nd₂, Nd_(4.4)Fe_(77.8)B_(17.8), NdFe₃(BO₃)₄, NdFeO₃, Fe, etc. The particle boundaries particularly new crack active surfaces formed at the step of compact molding should be covered entirely with the suppressing phase formed by heat treatment. It is an object of the present invention to provide a magnet bonded with a bonding material whose main phase is Nd₂Fe₁₄B with further improved magnetic characteristics and a method of manufacturing the magnet. Especially, the magnet with high reliability that allows using the magnet under severe environment of high humidity and high pressure is provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a TEM secondary electron image photograph showing a cross section structure of SiO₂ impregnated compact molding test piece.

FIG. 1B is an EDX spectrum of a mother phase.

FIG. 1C is an EDX spectrum of a grain boundary.

FIG. 2 A is a SEM image photograph of secondary electron of the test piece.

FIG. 2B is a SEM image photograph of oxygen atom analysis.

FIG. 2C is a SEM image photograph of Si atom analysis.

FIG. 3 is a graph showing demagnetization curves of SiO₂ impregnated heat treated compact molding and SiO₂ impregnated non-heat treated compact molding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As an example of the embodiment of the present invention, the magnet of the present invention comprises magnetic powder containing a rare earth element, a Fe rich phase (suppressing phase) having an atomic percentage of Fe higher than that of the magnetic powder, the Fe rich phase covering entirely or partially the surface of the magnetic powder and an inorganic binder that bonds the magnetic powder. In the present specification, the magnetic powder is used to mean particles of the magnetic powder, unless otherwise specified. For example, the Fe rich phase covers entirely or partially the surfaces of the particles of the magnetic powder.

Further, the Fe rich phase contains the rare earth element and oxygen. In addition, the magnetic powder comprises a main phase of Nd₂Fe₁₄B, and the Fe rich phase is an oxide containing Nd and Fe wherein an atomic percentage of Fe to Nd+Fe (×100) is 50% or more. The Fe rich phase contains at least one of Fe₁₇Nd₂, Nd₄₄Fe_(77.8)B_(17.8), NdFe₃(BO₃)₄, NdFeO₃ and Fe. The binder is SiO₂ having an alkoxy group. There is formed a high resistance film having a thickness of 10 nm to 10 μm between the surface of the magnetic powder and the inorganic binder.

The present invention provides a method of manufacturing a magnet, which comprises compact-molding magnet powder, which comprises at least Fe and a rare earth element to obtain a compact-molded magnet, heat-treating the compact-molded magnet to form a Fe rich phase having a Fe atomic percentage higher than that of the main phase of the magnetic powder (hereinafter referred to as a mother phase), impregnating a precursor solution of the inorganic binder material into the compact-molded magnet, and heat-treating the impregnated magnet thereby to cure the precursor in the precursor solution to form the binder material. Prior to the compact-molding the magnetic powder, a treating solution containing a fluoride compound is added to the magnetic powder and the magnetic powder is heat treated to form a fluoride coating on the surface of the magnetic powder.

According to the embodiments of the present invention, magnetic characteristics of the magnet bonded with the bonding material. Particularly, it is possible to provide a rare earth element magnet by covering newly formed cracks of the magnetic powder formed during compact-molding the magnetic powder with a high corrosion resistance film so that the magnet may keep high magnetic characteristics under severe environment.

In the following the embodiments of the present invention will be explained.

Example 1

Each of the steps for manufacturing the compact-molded magnet will be explained.

(1) Magnetic powder is prepared as follows. In this example as rare earth element magnetic powder, used was a thin foil of NdFeB alloy, which was prepared by rapidly quenching molten mother alloy. The NdFeB mother alloy was prepared by mixing Nd with Fe—B alloy (ferro boron, FeB) and melting the mixture in an inert gas atmosphere or a reducing gas atmosphere to produce the alloy with a homogeneous composition.

If desired, the molten alloy is formed into a thin foil on a rotating single roll or twin rolls by injecting the molten alloy and rapidly quenching in the inert gas such as argon gas or reducing gas atmosphere, followed by heat-treating the foil in the inert gas or in the reducing gas. The heat treating temperature is 200 to 700° C. The heat treatment causes fine grains of Nd₂Fe₁₄B to be formed in the magnetic powder and to grow therein. The thin foil has a thickness of 10 to 100 μm and fine crystal grains of Nd₂Fe₁₄B have a diameter of 10 to 100 nm. As an example of the magnetic powder, MQP-14-12 manufactured by Magnequench International Inc. can be used.

(2) The magnetic powder is compact-molded. For example, in manufacturing magnets for use in electric rotating machines it is possible to shape magnetic powder in accordance with a final shape of the permanent magnet for the electric rotating machine.

According to the following method, dimensions of compacted magnets do not change too much in following steps. Therefore, it is possible to produce magnets with high precision. That is, it is highly possible to realize high precision required for the permanent magnet type electric rotating machines. For example, a high precision required for magnets used for magnet-built-in electric rotating machines.

On the other hand, in conventional magnets a precision of magnets was very bad, which needs cutting and grinding work of the magnets. This means not only making the productivity worse, but also making the magnetic characteristics worse by cutting and grinding work.

In case an average size of fine crystals of Nd₂Fe₁₄B is 30 nm, an inter-grain-layer has a composition close to Nd₇₀Fe₃₀ or Fe precipitated in the layer, and a thickness of the layer is thinner than a critical grain size of a single magnetic segment so that magnetic barriers are hard to be formed in the crystals of Nd₂Fe₁₄B. It is presumed that magnetic reversal is caused mainly by exchange interaction of Nd₂Fe₁₄B fine grains that magnetically couple to each other or by magnetic dipole interaction among magnetic powder.

Ground magnetic powder is charged compacted in a super high-hardness metal mold with an upper and lower punches under a pressure of 5 to 20 t/cm² in a magnetic field perpendicular to the magnetic field so that the resulting compact molding has little non-magnetic portions.

Since particles of the magnetic powder, which are prepared by crushing thin foil have a flake shape, the compact molding has anisotropy of orientation of the flake shape particles, and a long axis of the flake shape particles is arranged in the direction perpendicular to the pressing direction, i.e. a direction parallel with a direction perpendicular to a thickness of the thin foil. Therefore, if the flake particle was one magnetic dipole, as a means for suppressing the magnetic reversal, the thickness of the non-magnetic portions should be made thin, thereby to magnetically couple the magnetic powder prepared by grinding the foil in the orientation direction, or the thickness of the non-magnetic portions should be made thick in a direction of the pressing (a direction perpendicular to the orientation). Further, since the long axis of the flake shape particles tends to be directed towards the pressing direction, magnetism in the direction perpendicular to the pressing direction is more continuous than in the pressing direction so that permeance of each of the particles becomes large and magnetic reversal becomes difficult.

From the above discussion, it is apparent that there is a difference in demagnetization curve between the pressing direction and a direction perpendicular to the pressing direction. In a compact molding of 10×10×10 cm³, when a demagnetization curve is measured after magnetization in 60 kOe in a direction perpendicular to the pressing direction, a residual magnetic flux density (Br) was 0.06 T and a coercive force (iHc) was 12.1 kOe; if in magnetization in a direction parallel with the pressing direction in 60 kOe, demagnetization curve is measured-along a direction of magnetization, Br was 0.06 T and iHc was 11.8 kOe. Accordingly, it is presumed that the difference in the demagnetization curves was caused by flake shape magnetic powder wherein the arrangement of the flakes has anisotropy in the molding.

Each particle has a size as small as 10 to 100 nm, and anisotropy of the crystalline orientation is very little, but if there is anisotropy in orientation of the flake particles, magnetic anisotropy may arise. In the case where anisotropic magnetic powder whose particles are granuled by arranging crystalline axis of the fine crystals, it is necessary to pre-sinter the molding using a non-magnetic mold in a magnetic field prior to pressing in a super-hardness mold, thereby to orient the magnetic particles (refer to Example 2).

In this example, the isotropic magnetic powder (1) is filled in the mold, and molded under a pressure of 16 t/cm². Compact molded test pieces of a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm for magnetic characteristics measurement and test pieces of a longitude of 15 mm, a width of 10 mm and a thickness of 2 mm for mechanical strength measurement were prepared.

(3) The magnetic powder is subjected to heat treatment in a reducing gas or an inert gas at a temperature range, which is higher than that employed for a bond magnet, but lower than a sintering temperature thereby to form a new face on part of the molded magnetic surface. It is necessary to cover entirely the inter-grain faces with newly formed phase that has effects of thermal demagnetization suppression and high anti-corrosion.

It is preferable to apply the heat treatment to the molding after the compact molding, which produces new crack faces. Forming the new cracks results in an increase in mechanical strength due to bonding strength among the magnetic powder particles.

The compact molding test pieces produced at (2) above was heat treated at the temperature mentioned-above under vacuum of 1×10⁻⁴ Torr or less for predetermined time period.

(4) After the compact molding, an SiO₂ precursor solution was impregnated into a molding having the new phase.

The precursor is a material having good wettability with the magnet molding. By impregnating the magnet molding with the bonding having good wettability with the molding, the surface of the magnet molding is covered with the bonding material to bond the magnetic powder particles. Because the solution enters minute portions by virtue of good wettability, excellent bonding effect is achieved by even a small amount of bonding material.

There are precursors of SiO₂ in the solution such as alkoxysilane. Compounds having alkoxy groups as side chains, which are represented by the formula 1 and formula 2 are examples.

As an organic solvent for the precursor, solvents that have the same main chain as that of the alkoxy group of the alkoxysiloxane or alkoxysilane, but the solvent are not limited thereto. Examples are methanol, ethanol, propanol, isopropanol, etc.

As a catalyst for hydrolysis and dehydration polymerization, there are acid catalysts, basic catalysts or neutral catalysts. Among the catalysts, the neutral catalysts are most suitable because they prevent corrosion of metals. As the neutral catalysts, organo-tin catalysts are preferable. Examples of the organo-tin catalysts are bis-(2-ethylhexanoate) tin, n-butyl tris (2-ethylhexanoate) tin, di-n-butyl bis(2-ethyl hexanoate) tin, di-n-butyl bis (2,4-pentane dionate) tin, di-n-butyl dilauryl tin, dimethyl di-neodecanoate tin, di-octyl di-laurylate tin, di octyl di-neodecanoate, etc. The catalysts are not limited thereto.

As acid catalysts, there are dilute hydrochloric acid, dilute sulfuric acid, dilute nitric acid, formic acid, acetic acid, etc. As basic catalysts, there are potassium hydroxide, sodium hydroxide, ammonia water, etc.

A total amount of alkoxysiloxane, alkoxysilane, their hydrolyzed products, and dehydrated polymers is 5 to 96% by volume of the compact molding. If the total amount of the alkoxysiloxane, alkoxysilane, their hydrolyzed products, and dehydrated polymers is less than 5 vol %, a strength of the bonding material after curing becomes insufficient because of small amount of the bonding material in the magnet. On the other hand, if the total amount exceeds 96 vol %, a viscosity of the bonding solution increases as polymerization of the SiO₂ precursor such as alkoxysiloxane or alkoxysilane proceeds. As a result, control of the viscosity of the bonding solution is difficult and impregnation of the solution is difficult.

Water and the precursor of SiO2 react to hydrolyze in accordance with the chemical reaction formula 1 and chemical reaction formula 2. The chemical formulae indicate partial hydrolysis of alkoxysiloxane or alkoxysilane.

An amount of water is one of factors for controlling the progress of hydrolysis of alkoxysiloxane or alkoxysilane. The hydrolysis reaction is important for mechanical strength of the bonding material after curing the bonding material. If the hydrolysis reaction does not take place, the polymerization of the hydrolyzed products do not take place, either. The dehydrated polymer is SiO₂. The SiO₂ exhibits high bonding strength with the magnetic powder and increases in mechanical strength of the magnet.

In addition, OH groups of silanol contribute to strong bonding with O atoms or OH groups on the surface of the magnet powder. However, if hydrolysis reaction proceeds to increase a concentration of silanol groups, dehydration polymerization between organic compounds containing silanol groups (hydrolyzed products of alkoxysiloxane or alkoxysilane) take place to increase a molecular weight of the organic compound thereby to increase a viscosity of the bonding material solution. This status of the solution is not proper for an impregnating solution.

Accordingly, it is important to add a proper amount of water to the precursor solution of alkoxysiloxane or alkoxysilane. That is, an additive amount of water for the hydrolysis reaction represented by the chemical formulae 4 and 5 should be 1/10 to 1 on the basis of reaction equivalents shown in the chemical formulae 4 and 5.

If the additive amount of water is less than 1/10 of the reaction equivalents shown in the chemical formulae 4 and 5, a concentration of silanol of the organic compounds is too small. As a result, the interaction between the organic compounds containing the silanol and the surface of the magnetic powder is too weak and dehydration polymerization hardly takes place.

A lot of alkoxy groups remain in SiO₂ of the reaction product, which leads to a lot of defects in SiO2 having poor strength.

On the other hand, if the additive amount of water is larger than 1 of the reaction equivalent in the chemical formulae 4 and 5, the organic compounds containing the silanol groups tend to occur hydration polymerization to increase a viscosity of the bonding material solution. As a result, dehydration polymerization of the solution hardly takes place. Accordingly, such the solution is not proper for impregnation.

As solvents for the bonding material solution, alcohols are used in general. Since alkoxy groups in the alkoxysiloxane quickly dissociate in the alcohol solvent, the alkoxy groups substitute with alcohol groups to keep equilibrium state. Accordingly, preferable alcohols are methanol, ethanol, n-propanol, iso-propanol, which have boiling points lower than water. However, though stability of solutions slightly low, water soluble solvent such as ketones including acetone can be employed as long as the viscosity of the solution does not increase within several hours and boiling point is lower than water.

In the following specifications of the treating solutions (bonding material solutions) of samples 1-15 are shown in Table 1.

TABLE 1 Sample Kind of Silicate Water Alcohol Tin dibutyl Viscosity No. SiO₂ precursor material alcohol compound (mL) (mL) (mL) dilaurylate (mL) (mPa · s) 1 CH₃O—Si(CH₃O)₂—O)m-CH₃ Methyl 5.0 0.96 95 0.05 1.8 Average of m: 4 alcohol 2 CH₃O—Si(CH₃O)₂—O)m-CH₃ Methyl 25 4.8 75 0.05 17 Average of m: 4 alcohol 3 CH₃O—Si(CH₃O)₂—O)m-CH₃ Methyl 100 3.84 0 0.05 80 Average of m: 4 alcohol 4 CH₃O—Si(CH₃O)₂—O)m-CH₃ Methyl 25 0.96 75 0.05 8.7 Average of m: 4 alcohol 5 CH₃O—Si(CH₃O)₂—O)m-CH₃ Methyl 25 4.8 75 0.05 17 Average of m: 4 alcohol 6 CH₃O—Si(CH₃O)₂—O)m-CH₃ Methyl 25 9.6 75 0.05 38 Average of m: 4 alcohol 7 CH₃O—Si(CH₃O)₂—OCH₃ Methyl 25 5.9 75 0.05 3.9 alcohol 8 CH₃O—Si(CH₃O)₂—O)m-CH₃ Methyl 25 4.8 75 0.05 17 Average of m: 4 alcohol 9 CH₃O—Si(CH₃O)₂—O)m-CH₃ Methyl 25 4.6 75 0.05 56 Average of m: 4 alcohol 10 CH₃O—Si(CH₃O)₂—OCH₃ Methyl 25 5.9 75 0.05 3.9 alcohol 11 C₂H₅O—Si(C₂H₅O)₂—OC₂H₅ Ethyl 25 4.3 75 0.05 2.6 alcohol 12 n-C₃H₇O—Si(n-C₃H₇O)₂—O-n-C₃H₇ Isopropyl 25 3.4 75 0.05 2.1 alcohol 13 CH₃O—Si(CH₃O)₂—O)m-CH₃ Methyl 25 9.6 75 0.05 23 Average of m: 4 alcohol 14 CH₃O—Si(CH₃O)₂—O)m-CH₃ Methyl 25 9.6 75 0.05 38 Average of m: 4 alcohol 15 CH₃O—Si(CH₃O)₂—O)m-CH₃ Methyl 25 9.6 75 0.05 92 Average of m: 4 alcohol

In this example, used was a precursor solution for a binder SiO2 contains 25 ml of CH₃O-{Si(CH₃O)₂—O}_(m)—CH₃ (m: 3-5, an average: 4), 4.8 ml of water, 75 ml of dehydrated methyl alcohol and 0.05 ml of tin dilaurylate dibutyl, and the solution was left at 25° C. for 2 days. A viscosity of the SiO₂ precursor solution was measured using Ostwald viscometer at 30° C.

The test piece of the compression molding prepared at (3) was placed in a vat in such a manner that the compression direction was on a horizontal direction. Then, the SiO₂ precursor solution was charged into the vat until the level of the solution comes above 5 mm from the top face of the test piece. The vat filled with the SiO2 precursor solution was set in a vacuum chamber, and the chamber was gradually evacuated to about 80 Pa. The vacuum chamber was left until bubbles do not occur from the test piece. The pressure of the vacuum chamber where the vat in which the SiO₂ precursor solution was filled and the test piece was set was gradually returned to normal pressure and the test piece was taken out from the SiO₂ precursor solution.

(5) By heat-treating the molding, a magnet using SiO₂ binder for magnetic powder was produced. The temperature for heat treatment was relatively low so that a shape and dimension of the magnet hardly changes at the heat treatment. Therefore, the dimension precision of the ultimate magnet was quite high. The test piece of the compact molding impregnated with the SiO₂ precursor solution at (4) was set in a vacuum drying furnace, and the test piece was subjected to vacuum drying treatment under 1 to 3 Pa at 150° C.

FIG. 1( a) is a transmission electron microscope (TEM) photograph of a cross section of the test piece according to the example of the present invention. FIGS. 1( b) and (c) are spectra of energy diffraction type (EDX) x-ray element analysis. These data was observation of cracks occurred by the compression molding.

FIG. 1 (b) is a secondary electron image which shows a mother phase of Nd₂Fe₁₄B, SiO₂ amorphous binder and a phase formed by high temperature heat treatment in the test piece. FIG. 1( b) EDX spectrum of the mother phase of Nd₂Fe₁₄B, and FIG. 1( c) is a spectrum of the phase formed at the grain boundary by high temperature heat treatment.

It is understood from FIG. 1( a) that the new phase was formed near grains of the mother phase of Nd₂Fe₁₄B, which are bonded by the SiO₂ binder.

It is understood from FIG. 1( c) that the new phase are formed from Fe₁₇Nd₂, Nd_(4.4)Fe_(77.8)B_(17.8), NdFe₃(BO₃)₄, NdFeO₃, Fe, etc. When compared with EDX analysis of FIG. 1( b), it is understood that the new phase comprises 50 atomic % or more of Fe; the new phase formed by the heat treatment is hereinafter called “Fe rich phase”.

TEM-EDX data of the SiO₂ bonded compact molding, which was not subjected to heat treatment at step (3) above, showed that a phase of oxidized mother phase, not the Fe rich phase, was observed. The heat treatment changes the composition of the grain boundaries.

FIGS. 2 (a), (b) and (c) show photographs of scanning type electron microscope (SEM) of a cross section of the compact molding. FIG. 2( a) is a secondary electron image, FIG. 2 (b) an in-plane oxygen analysis image, and FIG. 2 (c) an in-plane silicon analysis image. As shown in (a), flake particles are laminated anisotropically and cracks are observed in part. Oxygen atoms and silicon atoms were observed along cracks in the surface and inside of the flake particles. These cracks were formed at the time of compact molding. The cracks are void before impregnation. From this fact, it was understood that the SiO₂ precursor solution was impregnated into the cracks of the magnetic powder. An enlarged figure of the grain boundaries is shown in FIG. 1 (a).

The produced magnet was subjected to various measurements of characteristics.

A bending strength of the compact molding test piece having a longitude of 15 mm, a width of 10 mm and a thickness of 2 mm before impregnation of the SiO₂ precursor solution was 2 MPa or less, but after impregnation and heat treatment, the compact molding magnet exhibited 30 MPa or more. When an increased concentration SiO₂ precursor solution was impregnated, the bending strength was 100 MPa or more.

A specific resistivity of the compact molding test piece having a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm was measured by a four probes method. Compared with a sintered rare earth element magnet, a specific resistivity of the compact molding magnet exhibited about 10 times a specific resistivity of the sintered magnet; but, compared with the rare earth element compact bond type magnet, the specific resistivity was about 1/10 the specific resistivity of the latter. This property is not adverse to ordinary motors of a rotation number of 10000 rpm or less, wherein an eddy current is small.

To the test pieces whose specific resistivity was measured a pulse magnetic field of 60 kOe or less was applied in a 10 mm direction (i.e. a direction of the longitude or width of 10 mm). Magnetic characteristics in the direction were measured. Since the magnet was produced by compact molding, a residual magnetic flux density was higher by 20 to 30% than that of the resin bond magnet. The residual magnetic flux and coercive force of the magnet before SiO₂ precursor solution and the magnet after impregnation of the SiO2 precursor solution were almost the same. It was elucidated that SiO₂ impregnation and subsequent heat treatment did not give an influence on magnetic characteristics at room temperature.

The test piece whose specific resistivity was measured was magnetized in a 10 mm direction in the magnetic field of 60 kOe at step (5) to measure thermal demagnetization rate. The thermal demagnetization rate is defined as residual magnetic flux density loss at room temperature before and after holding the SiO₂ bond test piece at 250° C. for 1 hour. The SiO₂ bond test piece exhibited the residual magnetic flux density loss of 11%, but the SiO₂ bond test piece, which was not subjected to the treatment at the step (3), exhibited the residual magnetic flux density loss of 15%. On the other hand, the epoxy resin bond magnet, the residual magnetic flux density loss of 35%.

Further, the test piece which was kept at 250° C. for 1 hour was magnetized again in a pulse magnetic field of 60 kOe or more in the same direction as the first magnetization to measure the residual magnetic flux density loss. As a result, the SiO2 bond and twice magnetization test piece with high temperature treatment exhibited the residual magnetic flux density loss of 0%, and the SiO₂ bond and twice magnetization without heat treatment exhibited the residual magnetic flux density loss of 2%. On the other hand, the resin bond twice magnetization test piece exhibited the residual magnetic flux density loss of 4%.

From the above facts, it has been elucidated that the Fe rich phase has functions to reduce thermal demagnetization rate by preventing oxidation and corrosion of the magnetic powder. In addition, there was a larger difference in residual magnetic flux density loss between the SiO₂ bond twice magnetization with heat treatment and the SiO₂ bond twice magnetization without heat treatment than that in irreversible residual magnetic flux loss. From this fact, it is presumed that the phase containing Fe more than the mother phase suppresses spin alleviation by thermal fluctuation, i.e. increases anisotropic energy. This is an important discovery in Nd—Fe—B system magnets, which have low Curie points. It was confirmed that the suppression of thermal demagnetization was occurred more or less by protection of the magnetic powder with SiO₂.

FIG. 3 shows demagnetization curves of a SiO₂ impregnated compact molding test piece, which was subjected to a high temperature treatment in vacuum of 1×10⁻⁴ Torr or lower, and of a SiO₂ impregnated compact molding test piece, which was not subjected to the heat treatment. The test pieces were subjected to a high temperature, high pressure and high humidity (PCT) under conditions of 2 ata at 120° C. in a humidity of 100% for 100 hours. The magnets were magnetized in 60 kOe. In the figure, the curves show the results before and after the PCT tests, and a vertical line represents magnetization (M) and a coordinate represents magnetic field (H).

The test pieces were magnetized in the magnetic field of 60 kOe in 10 mm direction and the magnetization along the direction was measured. The PCT conditions are shown above. The test piece that has not been subjected to the high temperature heat treatment showed a coercive force of −35% the value of the test piece before PCT test (hereinafter the same comparison otherwise mentioned) and a residual magnetic flux density of −5.3%, but the test piece that has been subjected to the high temperature heat treatment showed a coercive force of −0.7% and a residual magnetic flux density of −3.5%. From these results, the Fe rich phase that was newly formed in the surface of the magnet powder by high temperature heat treatment works as a high corrosion resistance layer.

The SiO₂ bond compact powder magnet of which at least part of grain boundaries of the powder is covered with the high corrosion resistance phase fulfils the specification for magnets for use in high temperature, high pressure and high humidity environment. Therefore, the magnet may have a wide application. The resistance under the high temperature, high pressure and high humidity environment was occurred by protection of the magnetic powder with SiO₂.

The magnets of the present invention can be manufactured simply by compact-molding the magnetic powder wherein the steps (4) and (5) for bonding the magnetic powder with SiO₂ and removing the solvent may be omitted. By covering the newly formed crack faces formed by compact-molding with the Fe rich phase, it is possible to provide magnets with high corrosion resistance and controlled thermal demagnetization suppression effect.

Example 2

In this example a method of manufacturing compact molding test pieces using anisotropic magnetic powder will be explained each step.

(1) A example will be explained by using anisotropic magnetic powder wherein c-axis of tetragonal fine Nd₂Fe₁₄B crystals were oriented. The magnetic powder used in this Example differs from flake magnetic powder, which has magnetic anisotropy, used in Example 1. In this Example, used was magnetic powder whose particle size was 1 to 1000 μm, the powder having been subjected to hydrogenation-dehydrogenation treatment (HDDR). A coercive force of the magnetic powder was 16 kOe at room temperature. Instead of this magnetic powder, other anisotropic magnetic powders can be employed.

(2) The magnetic powder prepared at the step (1) was filled in a non-magnetic mold and pre-compact-molded under a pressure of 50 MPa in a magnetic field of 1.5 T. The anisotropic magnetic powder is oriented in the magnetic field. The pre-molding in the magnetic field may be useful for magnetic powder of shape anisotropy. The pre-molding was compacted under a pressure of 16 t/cm² and the compacted-molding was formed into test pieces each having a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm for measuring magnetic characteristics and test pieces each having a longitude of 15 mm, a width of 10 mm and a thickness of 2 mm for measuring mechanical properties. A direction of application of magnetic field at the time of the pre-molding was 10 mm.

(4) The test pieces prepared at the step (3) were placed in a vat in such a manner that a pressurizing direction of the molding was on a horizontal line. A SiO₂ precursor solution was charged into the vat at a rate of 1 mm/min in terms of rising speed of the liquid level. The solution was charged into the vat until the liquid level was 5 mm above the top surface of the test pieces. The SiO₂ precursor solution comprised 25 mL of CH₃O—(Si(CH₃O)₂—O)_(m)—CH₃ (m; 3-5, average; 4), 4.8 mL of water, 75 mL of dehydrated methyl alcohol, and 0.05 mL of dilaurylate dibutyl tin, the solution was left for two days at 25° C. Viscosity of the solution was measured with an Ostwald viscometer at 30′.

The vat in which the test pieces were placed and which was filled with the SiO₂ precursor solution was set in a vacuum chamber. The vacuum chamber was slowly evacuated to about 80 Pa. The reduced pressure was kept until bubbles did not occur in the surface of the test pieces. The vat was taken out from the vacuum chamber after the inner pressure of the vacuum chamber was slowly returned to an atmosphere. The test pieces were taken out from the SiO₂ precursor solution in the vat.

(5) The test pieces prepared at the step (4) were set in a vacuum drying furnace and were subjected to vacuum heat treatment under a pressure of 1-3 Pa at 150° C.

Crack faces that were formed by the compact-molding were observed with a transmission electron microscope (TEM) and subjected to energy distribution type X-ray (EDX) element analysis. As same as in example 1, it was confirmed that a new phase was formed on the magnetic powder, the new Fe rich phase comprising Fe₁₇Nd₂, Nd_(4.4)Fe_(77.8)B_(17.8), NdFe₃(BO₃)₄, NdFeO₃, Fe, etc. In the case where the test pieces that have not been subjected to the high temperature treatment at step (3), TEM-FED measurement showed that the Fe rich phase was not formed but the mother phase was oxidized. Accordingly, the compositions of the test pieces before and after were obviously changed.

The bending strength of the test pieces having a longitude of 15 mm, a width of 10 mm and a thickness of 2 mm before SiO₂ precursor solution impregnation was 2 MPa or less, but the bending strength of the SiO₂ precursor solution test pieces was 30 MPa or more, and the test pieces with which a high concentration SiO₂ precursor solution was impregnated showed 100 MPa or more.

Specific resistivity of the test pieces having a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm was measured by a four probe method of resistivity measurement. The test pieces of the example showed the specific resistivity of about ten times that of sintered type magnet, but the value was about 1/10 that of the compact molding type rare earth element bond magnet. However, there is no problem as long as the magnet of this example is used for ordinary motors whose rotation speed is 10000 rpm or less because the eddy current in the magnet is small.

The test pieces after measurement of the specific resistivity were magnetized in a pulse magnetic field in a 10 mm direction as at the pre-molding step. Magnetic characteristics of the resulting test pieces were measured. The compact molding magnet showed residual magnetic flux density 20 to 30% higher than that of the resin bond magnets.

Demagnetization curves at 20° C. showed that residual magnetic flux density and coercive force of the test pieces before and after the SiO₂ precursor solution impregnation were approximately the same. Thus, it was revealed that the SiO₂ precursor solution impregnation and subsequent heat treatment did not give influence on the magnetic characteristics.

Test pieces prepared at the step (5) that were magnetized in the pulse magnetic field of 60 kOe or more in 10 mm direction were subjected to thermal demagnetization rates. The thermal demagnetization is defined as residual magnetic flux density loss at room temperature before and after holding the test pieces at 250° C. for one hour. The SiO₂ bond test pieces with high temperature treatment showed the residual magnetic flux density loss of 11%, but the SiO₂ bond test pieces without high temperature treatment showed the residual magnetic flux density loss of 15%.

Irreversible residual magnetic flux density loss was measured after the test pieces were held at 250° C. for one hour and magnetized again in a magnetic field of 60 kOe or more. As a result, the test pieces of the example showed the irreversible residual magnetic flux density loss of 0%, but the test pieces that were not subjected to heat treatment showed 2%. From the above facts, it was elucidated that the new Fe rich phase formed by the heat treatment was effective for suppressing thermal demagnetization. This is an important discovery for the Nd—Fe—B magnets that have a low Curie point. The suppression of the thermal demagnetization was occurred by protection of the magnetic powder with SiO₂ coating.

A high temperature, high pressure, high humidity test (PCT) on the test pieces prepared at the step of (5) each having a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm was carried out. The test pieces were magnetized in a magnetic field of 60 kOe or more in 10 mm direction and the magnetization along the direction were measured. PCT conditions were temperature of 120° C., pressure of 2 ata, humidity of 100%, and magnetization time of 100 hours. It was observed that compared with the test pieces before the PCT test, the coercive force and residual magnetic flux density of the non-heat treated SiO2 bond test pieces were −35% and −5.3%, respectively, but the heat treated SiO₂ bond test pieces showed the coercive force and residual magnetic flux density of −0.7% and −3.5%, respectively. From these results, it was elucidated that the Fe rich phase formed on the surface of the magnetic powder by the high temperature heat treatment functioned as a high anti-corrosion phase.

The SiO₂ bond magnet at least part of the surface of the magnetic particles being covered with the high corrosion resistance phase will satisfy the specification of application under high temperature, high pressure and high humidity atmosphere, which may expand the application of the magnet.

It was confirmed that resistance to the high temperature, high pressure and high humidity atmosphere was occurred more or less by protection of the magnetic powder with SiO₂ coating.

Example 3

In this example, there are shown bending strength, specific resistivity and magnetic characteristics when compositions of the SiO₂ precursor solution were differently changed. As the magnetic powder, MQP-14-12 manufactured by Magnequench International Inc. was used. In the following, steps for manufacturing the magnets will be explained.

(1) MQP14-12, which is rapid-quenched magnetic powder, is chemically stable magnetic powder, which is not oxidized or corroded in atmosphere. The above-magnetic power is flake powder having a thickness of 10 to 100 μm.

(2) The powder of (1) was filled in a mold and molded under a pressure of 16 t/cm² into test pieces each having a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm for measuring magnetic characteristics.

(3) The compact molding test pieces prepared at the step of (2) were subjected to heat treatment at a temperature higher than the temperature employed in the bond magnet, but lower than that employed in the sintered magnet under 1×10⁻⁴ Torr for a suitable time.

(4) The test pieces prepared at the step of (3) were placed in a vat in a manner that the compaction direction of the test pieces is on a horizontal line, and a SiO₂ precursor solution was charged into the vat at a rate of 1 mm/min until the liquid level was 5 mm above the top face of the compact molding test pieces.

As the SiO₂ precursor solution, used were compositions No. 1-15 shown in Table 1. The vat in which the test pieces were placed and the SiO₂ precursor solution was filled was set in a vacuum chamber. The vacuum chamber was slowly evacuated to about 80 Pa. The test pieces were left until bubbles were not observed in the surface of the test pieces. The inner pressure of the vacuum chamber where the vat was set was slowly recovered to atmospheric pressure, and the test pieces were taken out from the SiO2 precursor solution.

(5) The compact molding test pieces prepared at the step of (4) were set in a vacuum drying furnace and they were subjected to vacuum heat treatment under a pressure of 1-3 Pa at 150 t.

Characteristics of the test pieces, i.e. bending strength, specific resistivity, coercive force and irreversible thermal demagnetization are shown in Table 2.

TABLE 2 Mechanical characteristics Magnetic characteristics of magnet of magnet Residual Irreversible Bending Specific magnetic Coercive thermal de- Sample strength resistivity flux force magnetization No. (MPa) (Ωcm) density (kG) (kOe) rate (%) 1 35 0.0017 7.1 12.2 <1 2 140 0.0019 6.8 12.2 <1 3 210 0.0025 6.7 12.2 <1 4 72 0.0016 6.9 12.2 <1 5 140 0.0019 6.8 12.2 <1 6 170 0.0031 6.7 12.2 <1 7 110 0.0021 6.9 12.2 <1 8 140 0.0019 6.9 12.2 <1 9 150 0.0019 6.8 12.2 <1 10 110 0.0021 6.9 12.2 <1 11 94 0.0020 6.9 12.2 <1 12 79 0.0019 7.0 12.2 <1 13 130 0.0035 6.8 12.2 <1 14 170 0.0031 6.7 12.2 <1 15 180 0.0029 6.7 12.2 <1

The samples Nos. 1-15 in table 2 correspond to samples Nos. 1-15 in Table 1.

The compact molding test pieces having a size of 15 mm×10 mm×2 mm were used for measuring bending strength wherein the bending strength was evaluated by a three point bending test with a fulcrum distance of 12 mm. Specific resistivity was measured by using test pieces having a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm by a four probe method. Magnetic characteristics were measured after magnetization at 60 kOe in 10 mm direction.

Example 4

In this example a process for making the magnetic powder high resistive will be explained. This process differs from that of examples 1 to 2 in that an insulating treatment is applied before compact molding of the magnetic powder to the magnetic powder. That is, the Fe rich phase formed by high temperature heat treatment and an insulating layer on the Fe rich phase or a new phase comprising mixed Fe rich phase and the insulating layer is formed around particles of the magnetic powder, and the particles are bonded with SiO₂ bond.

In this insulating treating step it is preferable to form the insulating layer homogeneously on the surface of the magnetic powder. Examples of the methods will be explained later.

When the magnets are used in electric rotating machines, for example, they are used in a saturate magnetic flux density state, in general. For example, in the electric rotating machines, magnetic flux formed by windings acts on the magnet and cyclically changes. When the magnetic flux changes, eddy current occurs in the magnet to thereby lower efficiency of the machines. By covering the surface of the magnet with an insulating phase, the eddy current is suppressed to prevent lowering of efficiency of the machines. In addition, under conditions where high frequency magnetic field containing high harmonic wave is applied to the magnet, the surface of the rare earth element magnet should preferably be covered with an inorganic insulating film.

Fluorides of rare earth elements or of alkaline earth metals in the treating solution for forming the insulating coating are swollen in a solvent whose min component is alcohol. The swollen fluorides of the rare earth elements or alkaline earth metals become gelatinous, which is soft and alcohol in the solution has good wettability with the rare earth element magnet powder. In addition, the alcohol solvent remarkably suppresses oxidation of the rare earth element magnet, which is easily oxidized.

In order to improve magnetic characteristics and insulating property of the magnetic powder, fluoride coat is preferable as the inorganic insulating film. Therefore, in forming the fluoride coat on the rare earth element magnet powder, a thickness of the coat on the rare earth element magnet depends on concentrations of rare earth element or alkaline earth elements or alkaline earth metals in the fluoride coat forming treating solution. Since the fluorides of rare earth metals or alkaline earth metals are swollen in the alcohol solvent, the fluorides of rare earth elements or alkaline earth metals are divided into an average size of several nm to 10 μm and the divided fluorides are suspended in the alcohol solvent. The concentration of the fluorides of rare earth elements or alkaline rare earth metals should be 1 to 200 g/dm³.

An additive amount of the fluoride coat forming solution depends on an average particle size of the rare earth element magnetic powder. When the average particle size of the rare earth element magnetic powder is 0.1 to 500 μm, the additive amount of the coat forming solution should be 10 to 300 mL. If the additive amount of the coat forming solution is over 300 mL, a time for removing the solvent needs a long time. On the other hand, if the additive amount is less than 10 mL, the surface of the magnetic powder may not be wetted sufficiently.

Table 3 shows compositions, status of solution, effective concentrations, solvents and an average particle size of metal fluorides for forming the fluorides of rare earth elements or fluorides of alkaline earth metals.

TABLE 3 Effective concentration for Average particle Treating component Status of solution treating solution Solvent size MgF2 Colorless, transparent ≦200 g/dm3 Methyl alcohol <100 nm CaF2 Milky, slightly viscous Do. Methyl alcohol <1000 nm LaF3 Semitransparent, viscous Do. Methyl alcohol <1000 nm LaF3 Milky, slightly viscous Do. Ethyl alcohol <2000 nm LaF3 Milky Do. n-propyl alcohol <3000 nm LaF3 Milky Do. Iso-propyl alcohol <5000 nm CeF3 Viscous, milky ≦100 g/dm3 Methyl alcohol <2000 nm PrF3 Yellow-greenish, semitransparent Do. Methyl alcohol <1000 nm NdF3 Light purple, semitransparent, ≦200 g/dm3 Methyl alcohol <1000 nm viscous SmF3 Milky Do. Methyl alcohol <5000 nm EuF3 Milky Do. Methyl alcohol <5000 nm GdF3 Milky Do. Methyl alcohol <5000 nm TbF3 Milky Do. Methyl alcohol <5000 nm DyF3 Milky Do. Methyl alcohol <5000 nm HoF3 Pink, milky ≦150 g/dm3 Methyl alcohol <5000 nm ErF3 Pink, milky, slightly viscous ≦200 g/dm3 Methyl alcohol <5000 nm TmF3 Slightly semitransparent, viscous Do. Methyl alcohol <1000 nm YbF3 Slightly semitransparent, viscous Do. Methyl alcohol <1000 nm LuF3 Slightly semitransparent, viscous Do. Methyl alcohol <1000 nm

In the following, isotropic magnetic powder was used. The following is applicable to anisotropic magnetic powder as well. In the latter vase, pre-molding in a magnetic field is necessary.

In this example, as the rare earth element magnetic powder, MQP-14-12 was used. Solutions for forming the coat film of fluorides of rare earth element or of alkaline earth metals were prepared in the following manner.

(1) 4 g of lanthanum acetate or lanthanum nitrate, which is well soluble in water was introduced into 100 mL of water and completely dissolved under stirring with a stirrer or an ultrasonic stirrer. (2) Hydro-fluoric acid of 10% was gradually added in an equivalent amount to form LaF₃. (3) Gelatinous precipitate solution of KaF₃ was stirred with an ultrasonic stirrer for one hour or more. (4) After centrifugation separation at 4000 to 6000 rpm, the supernatant was removed, and a same amount of methanol was added. (5) After the methanol solution containing gelatinous LaF₃ was stirred to make a suspension, the solution was stirred with the ultrasonic stirrer for one hour or more. (6) The steps (4) and (5) were repeated until acetate ions and nitrate ions are not detected. (7) An almost transparent gelatinous LaF₃ was obtained. As the fluoride coat film forming solution, a methanol solution of LaF3 of 1 g/5 mL was used.

Tables 4-1 and 4-2 show other materials, conditions of the fluoride coat forming solutions, and magnetic characteristics of the magnets produced by using the solutions.

TABLE 4-1 Magnet characteristics Magnet characteristics Condition for coating solution of fluoride (mechanical properties) (Magnetic properties) Additive amount of Specific Residual Coercive Irreversible thermal treating solution/100 g Concentration Bending strength resistance magnetic flux force demagnetization magnetic powder (mL) (g/dm3) solvent (MPa) (Ωcm) density (kG) (kOe) (%) MgF2 15 100 Methyl alcohol 130 0.032 6.6 12.2 <1 CaF2 15 100 ″ 100 0.026 6.5 12.2 <1 LaF3 15 100 ″ 120 0.03 6.5 12.3 <1 LaF3 15 100 Ethyl alcohol 97 0.027 6.4 12.5 <1 LaF3 15 100 n-propyl alcohol 76 0.025 6.5 12.3 <1 LaF3 15 100 Iso-propyl alcohol 54 0.021 6.6 12.3 <1 CeF3 15 100 Methyl alcohol 110 0.029 6.5 12.3 <1 PrF3 15 100 ″ 110 0.031 6.4 13.8 <1 NdF3 15 100 ″ 110 0.028 6.6 12.5 <1 SmF3 15 100 ″ 75 0.023 6.6 12.5 <1 EuF3 15 100 ″ 73 0.022 6.5 12.4 <1 GdF3 15 100 ″ 69 0.023 6.4 12.3 <1

TABLE 4-2 Magnet characteristics Magnet characteristics Condition for coating solution of fluoride (mechanical properties) (Magnetic properties) Additive amount of Bending Specific Residual Coercive Irreversible treating solution/100 g Concentration strength resistance magnetic force thermal demagnetization magnetic powder (mL) (g/dm3) solvent (MPa) (Ωcm) flux density (kG) (kOe) (%) TbF3 15 100 Methyl alcohol 70 0.025 6.4 18.9 <1 DyF3 15 100 ″ 68 0.026 6.3 18.5 <1 HoF3 15 100 ″ 57 0.024 6.4 12.6 <1 ErF3 15 100 ″ 52 0.021 6.5 12.5 <1 TmF3 15 100 ″ 56 0.023 6.5 12.9 <1 YbF3 15 100 ″ 53 0.025 6.4 12.2 <1 LuF3 15 100 ″ 50 0.027 6.1 12.3 <1 PrF3 1 10 ″ 130 0.018 6.3 13.1 <1 PrF3 10 10 ″ 120 0.018 6.5 13.5 <1 PrF3 30 10 ″ 120 0.018 6.4 13.6 <1 DyF3 10 1 ″ 130 0.017 6.5 13.5 <1 DyF3 10 10 ″ 110 0.017 6.6 15.5 <1 DyF3 10 200 ″ 42 0.036 6.5 18.5 <1

Example 5

In this example, as the rare earth element magnetic powder, MQP-14-12 was used, but other isotropic or anisotropic magnetic powders can be used. In case of the anisotropic magnetic powder, compact molding in magnetic field is necessary. The formation of fluorides of rare earth element or alkaline earth metal on Nd₂Fe₁₄B magnetic powder was carried out in the following manner.

This process used an NdF₃ semitransparent sol solution of a concentration of 1 g/10 mL for forming NdF₃ film.

(1) 15 mL of the NdF₃ sol solution was added to 100 g of magnetic powder that prepared by grinding foil of Nd₂Fe₁₄B, and the mixture was kneaded until the entire of the magnetic powder was wetted. (2) The rare earth magnetic powder treated with the NdF₃ coat solution at the step (1) was subjected to methanol removal treatment under a reduced pressure of 2 to 5 Torr. (3) The rare earth element magnetic powder prepared at the step (2) was charged in a quartz boat and was subjected to heat treatment under a pressure of 1×10⁻⁵ at a temperature of 200□ for 30 min and at 400° C. for 30 min.

The Nd₂Fe₁₄B magnetic powder treated with the NdF₃ film forming solution was charged in a mold, and was formed under a pressure of 16 t/cm² into test pieces for measuring magnetic characteristics each having a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm, and test pieces for measuring mechanical strength each having a longitude of 15 mm, a width of 10 mm and a thickness of 2 mm.

The resulting test pieces were heat treated at a temperature higher than a temperature employed in the process for manufacturing the ordinary bond magnet, but lower than a temperature employed for manufacturing sintered magnets under a pressure of 1×10⁻⁴. The resulting test pieces were placed in a vat in a manner that the pressing direction was on the horizontal line, and SiO₂ precursor solution was charged in the vat at a rate of 1 mm/min until the liquid level became 5 mm above the top face of the test pieces. As the SiO2 bond material, 25 mL of CH₃O—(Si(CH₃O)₂—O)_(m)—CH₃ (m: 3-5. average: 4), 4.8 mL of water, 75 mL of dehydrated methyl alcohol and 0.05 mL of dilaurate dibutyl tin were mixed, and the mixture was left for two days at 25□.

The vat charged with the SiO₂ precursor solution was set in a vacuum chamber which was evacuated slowly to about 80 Pa until bubbles are not observed in the surfaces of the test pieces. After the inner pressure of the vacuum chamber was returned slowly to the normal pressure, the test pieces were taken out from the SiO₂ precursor solution in the vat. Finally, the test pieces were set in a vacuum drying furnace, and the test pieces were subjected to vacuum heat treatment under a pressure of 1-3 Pa at 150° C.

The new crack faces were observed with TEM and analyzed by EDX. As in the same in the example 1, the new crack faces were Fe rich phase containing Fe₁₇Nd₂, Nd_(4.4)Fe_(77.8)B_(17.8), NdFe₃(BO₃)₄, NdFeO₃, Fe, etc. At the same time, mixed crystals of NdF₃ or mixed compounds with these phases and oxy-fluoride phases were observed. By the high temperature heat treatment, it is presumed that part of NdF₃ flew into the crack faces.

Test pieces each having a longitude of 15 mm, a width of 10 mm and a thickness of 2 mm were subjected to bending strength tests by the three point bending test method wherein a distance between fulcrums was 12 mm. Though the bending strength of a test piece before SiO₂ precursor impregnation was 2 MPa or less, the test piece that has been impregnated with the SiO₂ precursor solution showed 60 MPa or more.

The test pieces having a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm were subjected to specific resistivity tests. Compared with sintered rare earth element magnets, the test pieces exhibited specific resistivity of 100 times or more, and showed values similar to that of compact rare earth element magnets. Therefore, the magnets of the example have a little eddy current and excellent magnetic characteristics.

The test pieces that have been subjected to the specific resistivity measurement were magnetized by pulse magnetic field of 60 kOe or more in a 10 mm direction. The test pieces were subjected to magnetic characteristic measurement. The magnetic characteristics of the test pieces exhibited residual magnetic flux density of 20 to 30% higher than the resin bond magnets on the market. The demagnetization curves at 20□ of the test pieces before and after SiO₂ precursor solution impregnation showed approximately same residual magnetic flux density and coercive force.

The test pieces were magnetized again in 10 mm direction after the magnetic characteristics at room temperature, and thermal demagnetization rates were measured. The thermal demagnetization rate is defined as residual magnetic flux density loss at room temperature before and after holding the test pieces at 250° C. for one hour. The SiO₂ bond test pieces with high temperature treatment showed the residual magnetic flux density loss of 10%, but the SiO₂ bond test pieces without high temperature treatment showed the residual magnetic flux density loss of 14%.

Irreversible residual magnetic flux density loss was measured after the test pieces were held at 250° C. for one hour and magnetized again in a magnetic field of 60 kOe or more. As a result, the test pieces of the example showed the irreversible residual magnetic flux density loss of 0%, but the test pieces that were not subjected to heat treatment showed 2%. From the above facts, it was elucidated that the new Fe rich phase formed by the heat treatment was effective for suppressing thermal demagnetization. This is an important discovery for the Nd—Fe—B magnets that have a low Curie point. The suppression of the thermal demagnetization was occurred by protection of the magnetic powder with SiO₂ coating.

A high temperature, high pressure, high humidity test (PCT) on the test pieces was carried out. It was observed that compared with the test pieces before the PCT test, the coercive force and residual magnetic flux density of the non-heat treated SiO₂ bond test pieces were −30% and −5.0%, respectively, but the heat treated SiO₂ bond test pieces showed the coercive force and residual magnetic flux density of −0.5% and −3.0%, respectively. From these results, it was elucidated that the Fe rich phase formed on the surface of the magnetic powder by the high temperature heat treatment functioned as a high anti-corrosion phase.

The SiO₂ bond magnet of which at least part of the surface of the magnetic particles is covered with the high corrosion resistance phase will satisfy the specification of application under high temperature, high pressure and high humidity atmosphere, which may expand the application of the magnet.

It was confirmed that resistance to the high temperature, high pressure and high humidity atmosphere was occurred more or less by protection of the magnetic powder with SiO₂ coating. The magnet exhibited withstanding to salt water spray test.

The fluoride coat film of rare earth elements or alkaline earth metals formed on the rare earth element magnet powder functions not only as an insulating film, but also as to contribute to improvement of coercive force of the magnet if PrF₃ is used as the coat film forming material.

From the results of the example, it was revealed that the bond magnets of the example, which were prepared by impregnating the SiO₂ precursor solution into the cold molding rare earth element magnet exhibited magnetic properties of about 20% better, a bending strength of 2 to 4 times higher and a irreversible thermal demagnetization of ¼ or less those of conventional resin bond rare earth element magnets. Further, high reliability of the magnets was achieved. In addition, when TbF₃ and DyF₃ are used as the coat film, great improvement of the magnetic characteristics was achieved.

Example 6

In this example, as the rare earth element magnetic powder, MQP-14-12 was used, but other isotropic or anisotropic magnetic powders can be used. In case of the anisotropic magnetic powder, compact molding in magnetic field is necessary. The formation of fluorides of rare earth element or alkaline earth metal on Nd₂Fe₁₄B magnetic powder was carried out in the following manner.

This process used a PrF₃ semitransparent sol solution of a concentration of 0.1 g/10 mL for forming PrF₃ film.

(1) 15 mL of the PrF₃ sol solution was added to 100 g of magnetic powder that prepared by grinding foil of Nd₂Fe₁₄B, and the mixture was kneaded until the entire of the magnetic powder was wetted. (2) The rare earth magnetic powder treated with the PrF₃ coat solution at the step (1) was subjected to methanol removal treatment under a reduced pressure of 2 to 5 Torr. (3) The rare earth element magnetic powder prepared at the step (2) was charged in a quartz boat and was subjected to heat treatment under a pressure of 1×10⁻⁵ at a temperature of 200° C. for 30 min and at 400° C. for 30 min.

The Nd₂Fe₁₄B magnetic powder treated with the PrF₃ film forming solution was charged in a mold, and was formed under a pressure of 16 t/cm² into test pieces for measuring magnetic characteristics each having a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm, and test pieces for measuring mechanical strength each having a longitude of 15 mm, a width of 10 mm and a thickness of 2 mm.

The resulting test pieces were heat treated at a temperature higher than a temperature employed in the process for manufacturing the ordinary bond magnet, but lower than a temperature employed for manufacturing sintered magnets under a pressure of 1×10⁻⁴. The resulting test pieces were placed in a vat in a manner that the pressing direction was on the horizontal line, and SiO₂ precursor solution was charged in the vat at a rate of 1 mm/min until the liquid level became 5 mm above the top face of the test pieces. As the SiO₂ bond material, 25 mL of CH₃O—(Si(CH₃O)₂—O)_(m)—CH₃ (m: 3-5. average: 4), 4.8 mL of water, 75 mL of dehydrated methyl alcohol and 0.05 mL of dilaurate dibutyl tin were mixed, and the mixture was left for two days at 25° C.

The vat charged with the SiO2 precursor solution was set in a vacuum chamber, which was evacuated slowly to about 80 Pa until bubbles are not observed in the surfaces of the test pieces. After the inner pressure of the vacuum chamber was returned slowly to the normal pressure, the test pieces were taken out from the SiO₂ precursor solution in the vat. Finally, the test pieces were set in a vacuum drying furnace, and the test pieces were subjected to vacuum heat treatment under a pressure of 1-3 Pa at 150° C.

The new crack faces were observed with TEM and analyzed by EDX. As in the same in the example 1, the new crack faces were Fe rich phase containing Fe₁₇Nd₂, Nd_(4.4)Fe_(77.8)B_(17.8), NdFe₃(BO₃)₄, NdFeO₃, Fe, etc. At the same time, mixed crystals of NdF₃ or mixed compounds with these phases and oxy-fluoride phases were observed. By the high temperature heat treatment, it is presumed that part of NdF₃ flew into the crack faces.

Test pieces each having a longitude of 15 mm, a width of 10 mm and a thickness of 2 mm were subjected to bending strength tests by the three point bending test method wherein a distance between fulcrums was 12 mm. Though the bending strength of a test piece before SiO₂ precursor impregnation was 2 MPa or less, the test piece that has been impregnated with the SiO₂ precursor solution showed 100 MPa or more.

The test pieces having a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm were subjected to specific resistivity tests. Compared with sintered rare earth element magnets, the test pieces exhibited specific resistivity of 100 times or more, and showed values similar to that of compact rare earth element magnets. Therefore, the magnets of the example have small eddy current and excellent magnetic characteristics.

The test pieces that have been subjected to the specific resistivity measurement were magnetized by pulse magnetic field of 60 kOe or more in a 10 mm direction. The test pieces were subjected to magnetic characteristic measurement. The magnetic characteristics of the test pieces exhibited residual magnetic flux density of 20 to 30% higher than the resin bond magnets on the market. The demagnetization curves at 20□ of the test pieces before and after SiO₂ precursor solution impregnation showed approximately same residual magnetic flux density and coercive force.

The test pieces were magnetized again in 10 mm direction after the magnetic characteristics at room temperature, and thermal demagnetization rates were measured. The thermal demagnetization rate is defined as residual magnetic flux density loss at room temperature before and after holding the test pieces at 250° C. for one hour. The SiO₂ bond test pieces with high temperature treatment showed the residual magnetic flux density loss of 9%, but the SiO₂ bond test pieces without high temperature treatment showed the residual magnetic flux density loss of 13%.

Irreversible residual magnetic flux density loss was measured after the test pieces were held at 250° C. for one hour, and magnetized again in a magnetic field of 60 kOe or more. As a result, the test pieces of the example showed the irreversible residual magnetic flux density loss of 0%, but the test pieces that were not subjected to heat treatment showed 2%. From the above facts, it was elucidated that the new Fe rich phase formed by the heat treatment was effective for suppressing thermal demagnetization. This is an important discovery for the Nd—Fe—B magnets that have a low Curie point. The suppression of the thermal demagnetization was occurred by protection of the magnetic powder with SiO₂ coating.

The PrF₃ coat film of rare earth elements or alkaline earth metals formed on the rare earth element magnet powder functions not only as an insulating film, but also as to contribute to improvement of coercive force of the magnet.

A high temperature, high pressure, high humidity test (PCT) on the test pieces was carried out. PCT conditions were: temperature was 120° C., pressure 2 ata, humidity 100%, and time 100 hours. It was observed that compared with the test pieces before the PCT test, the coercive force and residual magnetic flux density of the non-heat treated SiO₂ bond test pieces were −30% and −5.0%, respectively, but the heat treated SiO₂ bond test pieces showed the coercive force and residual magnetic flux density of −0.5% and −3.0%, respectively. From these results, it was elucidated that the Fe rich phase formed on the surface of the magnetic powder by the high temperature heat treatment functioned as a high anti-corrosion phase.

The SiO₂ bond magnet at least part of the surface of the magnetic particles being covered with the high corrosion resistance phase will satisfy the specification of application under high temperature, high pressure and high humidity atmosphere, which may expand the application of the magnet.

It was confirmed that resistance to the high temperature, high pressure and high humidity atmosphere was occurred more or less by protection of the magnetic powder with SiO₂ coating. The magnet exhibited withstanding to salt water spray test.

From the results of the example, it was revealed that the bond magnets of the example, which were prepared by impregnating the SiO₂ precursor solution into the cold molding rare earth element magnet exhibited magnetic properties of about 20% better, a bending strength of 3 to 4 times higher and a irreversible thermal demagnetization of ¼ smaller than those of conventional resin bond rare earth element magnets. The rare earth element magnets having the PrF₃ coat have well balanced magnetic characteristics, bending strength and reliability.

Example 7

In this example, as the rare earth element magnetic powder, MQP-14-12 was used, but other isotropic or anisotropic magnetic powders can be used. In case of the anisotropic magnetic powder, compact molding in magnetic field is necessary. The formation of fluorides of rare earth element or alkaline earth metal on Nd2Fe14B magnetic powder was carried out in the following manner.

This process used a DyF₃ semitransparent sol solution of a concentration of 2 to 0.01 g/10 mL for forming NdF₃ film.

(1) 10 mL of the DyF₃ sol solution was added to 100 g of magnetic powder that prepared by grinding foil of Nd₂Fe₁₄B, and the mixture was kneaded until the entire of the magnetic powder was wetted. (2) The rare earth magnetic powder treated with the DyF₃ coat solution at the step (1) was subjected to methanol removal treatment under a reduced pressure of 2 to 5 Torr. (3) The rare earth element magnetic powder prepared at the step (2) was charged in a quartz boat and was subjected to heat treatment under a pressure of 1×10⁻⁵ at a temperature of 200° C. for 30 min and at 400° C. for 30 min.

The Nd₂Fe₁₄B magnetic powder treated with the NdF₃ film forming solution was charged in a mold, and was formed under a pressure of 16 t/cm² into test pieces for measuring magnetic characteristics each having a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm, and test pieces for measuring mechanical strength each having a longitude of 15 mm, a width of 10 mm and a thickness of 2 mm.

The resulting test pieces were heat treated at a temperature higher than a temperature employed in the process for manufacturing the ordinary bond magnet, but lower than a temperature employed for manufacturing sintered magnets under a pressure of 1×10⁻⁴. The resulting test pieces were placed in a vat in a manner that the pressing direction was on the horizontal line, and SiO₂ precursor solution was charged in the vat at a rate of 1 mm/min until the liquid level became 5 mm above the top face of the test pieces. As the SiO₂ bond material, 25 mL of CH₃O—(Si(CH₃O)₂—O)_(m)—CH₃ (m: 3-5. average: 4), 4.8 mL of water, 75 mL of dehydrated methyl alcohol and 0.05 mL of dilaurate dibutyl tin were mixed, and the mixture was left for two days at 25′.

The vat charged with the SiO2 precursor solution was set in a vacuum chamber, which was evacuated slowly to about 80 Pa until bubbles are not observed in the surfaces of the test pieces. After the inner pressure of the vacuum chamber was returned slowly to the normal pressure, the test pieces were taken out from the SiO₂ precursor solution in the vat. Finally, the test pieces were set in a vacuum drying furnace, and the test pieces were subjected to vacuum heat treatment under a pressure of 3 Pa at 150° C.

The new crack faces were observed with TEM and analyzed by EDX. As in the same in the example 1, the new crack faces were Fe rich phase containing Fe₁₇Nd₂, Nd_(4.4)Fe_(77.8)B_(17.8), NdFe₃(BO₃)₄, NdFeO₃, Fe, etc. At the same time, mixed crystals of DyF₃ or mixed compounds with these phases and oxy-fluoride phases were observed. By the high temperature heat treatment, it is presumed that part of DyF₃ flew into the crack faces. Further, an ion exchange between Nd in the mother phase and Dy in the fluoride was observed wherein Dy atoms migrated in the vicinity of the grain boundaries.

Test pieces each having a longitude of 15 mm, a width of 10 mm and a thickness of 2 mm were subjected to bending strength tests by the three point bending test method wherein a distance between fulcrums was 12 mm. Though the bending strength of a test piece before SiO₂ precursor impregnation was 2 MPa or less, the test piece that has been impregnated with the SiO₂ precursor solution showed 50 MPa or more.

The test pieces having a longitude of 10 mm, a width of 10 mm and a thickness of 8 mm were subjected to specific resistivity tests. Compared with sintered rare earth element magnets, the test pieces exhibited specific resistivity of 100 times or more, and showed values similar to that of compact rare earth element magnets. Therefore, the magnets of the example have small eddy current and excellent magnetic characteristics.

The test pieces that have been subjected to the specific resistivity measurement were magnetized by pulse magnetic field of 60 kOe or more in a 10 mm direction. The test pieces were subjected to magnetic characteristic measurement. The magnetic characteristics of the test pieces exhibited residual magnetic flux density of 20 to 30% higher than the resin bond magnets on the market. The demagnetization curves at 20□ of the test pieces before and after SiO₂ precursor solution impregnation showed approximately same residual magnetic flux density and coercive force.

The test pieces were magnetized again in 10 mm direction after the magnetic characteristics at room temperature, and thermal demagnetization rates were measured. The thermal demagnetization rate is defined as residual magnetic flux density loss at room temperature before and after holding the test pieces at 250° C. for one hour. The SiO₂ bond test pieces with high temperature treatment showed the residual magnetic flux density loss of 8%, but the SiO₂ bond test pieces without high temperature treatment showed the residual magnetic flux density loss of 12%.

Irreversible residual magnetic flux density loss was measured after the test pieces were held at 250° C. for one hour, and magnetized again in a magnetic field of 60 kOe or more. As a result, the test pieces of the example showed the irreversible residual magnetic flux density loss of 0%, but the test pieces that were not subjected to heat treatment showed 1%. From the above facts, it was elucidated that the new Fe rich phase formed by the heat treatment was effective for suppressing thermal demagnetization. This is an important discovery for the Nd—Fe—B magnets that have a low Curie point. The suppression of the thermal demagnetization was occurred by protection of the magnetic powder with SiO₂ coating.

A high temperature, high pressure high humidity test (PCT) on the test pieces was carried out. PCT conditions were: temperature was 120° C., pressure 2 ata, humidity 100%, and time 100 hours. It was observed that compared with the test pieces before the PCT test, the coercive force and residual magnetic flux density of the non-heat treated SiO₂ bond test pieces were −30% and −5.0%, respectively, but the heat treated SiO₂ bond test pieces showed the coercive force and residual magnetic flux density of −0.5% and −3.0%, respectively. From these results, it was elucidated that the Fe rich phase formed on the surface of the magnetic powder by the high temperature heat treatment functioned as a high anti-corrosion phase.

The SiO₂ bond magnet at least part of the surface of the magnetic particles being covered with the high corrosion resistance phase will satisfy the specification of application under high temperature, high pressure and high humidity atmosphere, which may expand the application of the magnet.

It was confirmed that resistance to the high temperature, high pressure and high humidity atmosphere was occurred more or less by protection of the magnetic powder with SiO₂ coating. The magnet exhibited withstanding to salt water spray test.

The DyF₃ coat film of rare earth elements or alkaline earth metals formed on the rare earth element magnet powder functions not only as an insulating film, but also as to contribute to improvement of coercive force of the magnet.

From the results of the example, it was revealed that the bond magnets of the example, which were prepared by impregnating the SiO₂ precursor solution into the cold molding rare earth element magnet exhibited magnetic properties of about 20% better, a bending strength of 2 to 4 times higher and a irreversible thermal demagnetization of ¼ smaller than those of conventional resin bond rare earth element magnets. The high reliability of the magnets was achieved. In addition, when TbF₃ and DyF₃ were used as the coat film, magnetic characteristics could be greatly improved.

The present invention has been explained by reference to examples. The magnets of the present invention have the following features.

1) The magnets keep high magnetic characteristics even in the severe environmental circumstance such as high temperature, high pressure, high humidity. 2) Performance of the magnets is better than that of the conventional resin bond magnets. 3) In addition to the excellent magnetic characteristics, the magnets have a mechanical strength higher than the conventional resin bond magnets.

The feature 1) will be achieved by the following manners has been explained.

The magnetic powder produces crack new faces when compact-molded. Since the new crack faces are active, the compact molding magnet has poor reliability. Particularly, under a severe environmental circumstance of the high temperature, high pressure and high humidity, magnetic characteristics of the magnet are drastically degraded. This problem may be solved by forming a film on the crack new faces.

For example, the compact molding is heat treated at a temperature higher than that employed for manufacturing the resin bond magnet, but lower than a sintering temperature so that a phase having an effect for suppressing thermal demagnetization is formed on part of the compact molding. This phase had an effect for increasing anisotropy of the magnetic powder.

The features 2) and 3) are achieved by the following manner.

It is necessary to fill gaps of 1 μm or less occurred between magnetic powder particles during compact molding with a bonding solution, i.e. a Si precursor solution. In order to carry out the impregnation, the solution should have a viscosity of 100 mPa·s or less and have good wettability with the magnetic powder. In addition, important things are strong bonding between the bonding material and the magnetic powder, and a high mechanical strength of the bonding material, and a continuous formation of the bonding material.

The viscosity of the bonding material solution may be chosen in accordance with a size of the magnets. When the thickness of the compact molding is 5 mm or less, and when the gaps are about 1 μm, the viscosity should be 100 mPa·s or less. If the thickness of the compact molding is 5 mm or more and the gaps are about 1 μm, 100 mPa·s is too high for impregnating a COMPACT molding having a thickness of 30 mm or more. Accordingly, the viscosity should be 20 mPa·s or less, preferably 10 mPa·s or less.

This viscosity is one tenth or less than that of conventional low viscosity impregnating resins for bond magnets, which has normally a viscosity of 200 to 400 mPa·s. In order to achieve the low viscosity, a control of an amount of hydrolysis of alkoxy groups in the alkoxysiloxane, a SiO₂ precursor, and control of a molecular weight of the alkoxysiloxane. That is, the alkoxy groups hydrolyze to silanol groups, which tend to dehydration polymerization reaction to form polymerized alkoxysiloxane with a high molecular weight. Further, the silanol groups generate hydrogen bonds to increase the viscosity of the alkoxysiloxane solution.

For example, addition of water in an equivalent amount to hydrolysis reaction of the alkoxysiloxane and control of hydrolysis reaction condition are essential. As a solvent for the SiO₂ precursor solution, i.e. SiO₂ bonding material, alcohols are preferable because the alkoxy groups in the alkoxysiloxane are quickly dissociated. The alcohols should have a boiling point lower than that of water, and preferably include methanol, ethanol, n-propanol and iso-propanol. As long as the viscosity of the SiO₂ bond solution does not increase within several hours and the boiling point is lower than that of water, such solvents may be employed in the production of the magnets of the present invention.

Regarding adhesion between the magnetic powder and the bonding material after curing, since the bonding material is SiO₂ after the heat treatment of the SiO₂ precursor, adhesion between the magnetic powder surface and the SiO₂, if the surface is covered with oxides that are formed by natural oxidation. Thus, if the magnet is ruptured, the surface is composed of aggregated destructive surfaces of SiO₂ and magnetic powder.

On the other hand, when resin is used as the bonding material, the adhesion between the resin and the magnetic powder is generally smaller than that between the magnetic powder surface and the SiO₂. For this reason, there are boundaries between the resin and the magnetic powder particles or aggregated destructive surface in the destructed surface of the resin bond magnet. Accordingly, the SiO₂ bond material is more suitable for bonding magnetic powder to increase mechanical strength of the magnets.

When a content of the magnetic powder in the rare earth element magnet is 75 vol % or more, rare earth element magnetic powder is used. The strength of the magnet after curing the bonding material greatly depends on whether a continuous body of the bonding material is formed or not, because the strength of the bonding material itself has higher than the fracture strength of the adhesion interface.

If an epoxy resin is used in an amount of 15 vol % per the total volume of the solid component of the compact molding, the resin does not become continuous but becomes distributed as islands in the magnet because of poor wettability between the resin and the rare earth element magnetic powder. On the other hand, the SiO₂ precursor continuously spreads over the surfaces of the particles of the magnetic powder because of the good wettability. The SiO₂ precursor continuously spreaded over the surfaces of the particles is cured to become SiO₂. On the other hand, SiO₂ has a bending strength higher by one to three powers than that of resin. Therefore, the strength of magnets using SiO₂ precursor as a bonding material is remarkably higher than that using the resin bonding material.

Suitable materials for the magnets of the present invention will be explained in the following.

Rare earth element magnets are made from a main phase of ferromagnetism and other components. When the rare earth element magnet is Nd—Fe—B system magnet, the main phase should be Nd₂Fe₁₄B. In considering the improvement of magnetic characteristics, magnetic powder should preferably be prepared by the HDDR method or magnetic powder prepared by a thermal plastic rolling method. As rare earth magnets, there are Sm—Co system magnets besides Nd—Fe—B system magnets. Taking into consideration the magnetic characteristics and production cost, Nd—Fe—B system magnets are preferable. However, the present invention is not limited to Nd—Fe—B magnets. If necessary, the Nd—Fe—B system magnetic powder may be mixed with different types of magnetic powders. For example, there may be used Nd—Fe—B system magnetic powders having different compositions or a mixture of Nd—Fe—B system magnetic powder and Sm—Co system magnetic powder.

In the present specification, “Nd—Fe—B system magnet” includes magnets wherein part of Nd or Fe is substituted with other elements. Nd may be substituted with other rare earth elements such as Dy, Tb, etc. The other elements may be used singly or in combination. The substitution of the elements may be carried out by adjusting compositions of alloy materials.

The substitution may bring about improvement of coercive force of Nd—Fe—B system magnets. An amount of Nd to be substituted is preferably 0.01 to 50 atomic % of Nd. If the amount is less than 0.01 atomic %, effects of substitution are insufficient, and if the amount of substitution is larger than 50 atomic %, there may be difficult to maintain the residual magnetic flux density with high level. Accordingly, the substitution should be chosen in accordance with uses.

On the other hand, Fe may be substituted with transition metals such as Co. By these substitutions, it is possible to increase curie point (Tc) of Nd—Fe—B system magnets thereby to widen the use temperature thereof. An amount of Fe to be substituted should be 0.01 to 30 atomic % of Fe. If the amount exceeds 30 atomic %, coercive force will be lowered. Therefore, the amount should be chosen considering the uses.

An average particle size of the rare earth element magnetic powder is preferably 1 to 500 μm. If the average particle size is smaller than 1 μm, the surface area of the powder is too large, which leads to oxidation and degradation of the surface of the powder. As a result, characteristics of the magnets would become worse.

On the other hand, if the average particle size of the magnetic powder is larger than 500 μm, the magnetic powder particle may be crushed at the time of compact molding, and a sufficient electric resistivity would not be obtained. In addition, in the case where anisotropic magnets are produced from anisotropic rare earth element magnetic powder, it is difficult to orient the main phase (in Nd—Fe—B system magnets, Nd₂Fe₁₄B phase) of the magnetic powder over particles larger than 500 μm. The average particle size of the magnetic powder is controlled by particle size of raw materials. The average particle size of the magnetic powder is calculated by SEM images.

The present invention can be applied to production of isotropic magnets produced from isotropic magnetic powder, isotropic magnets wherein anisotropic magnetic powder is oriented randomly, anisotropic magnets wherein anisotropic magnetic powder is oriented in one direction. If a magnet having a high energy product is needed, anisotropic magnetic powder is oriented in a magnetic field to produce anisotropic magnet.

The rare earth element magnets are produced by mixing raw materials in accordance with compositions for the magnets to be produced. If an Nd—Fe—B system magnet whose main phase is Nd₂Fe₁₄B is produced, Nd, Fe and B are mixed in predetermined amounts. The rare earth element magnetic powder can be prepared by conventional methods or materials purchased on the market can be used.

The particles of the magnetic powder comprise agglomerate of a number of crystal grains. If the crystal grains constituting the rare earth element magnetic powder have a particle size smaller than that of unit magnetic segment particles, coercive force will be increased. For example, the crystal gains should have an average particle size of 500 μm or less.

The HDDR method is a method for producing Nd₂Fe₁₄B wherein Nd—Fe—B system alloy is hydrided at first, thereby to decompose the main phase Nd2Fe14B into three phases of NdH₃, α-Fe and Fe₂B and the three phases are subjected to dehydrogenation to form Nd₂Fe₁₄B again. The UPSET method is a method wherein Nd—Fe—B system alloy produced by rapid quenching is crushed and pre-molded, followed by heat plastic rolling.

Magnetic powder of the magnets that are used under high frequency including harmonic wave conditions should be covered with an inorganic insulating film. That is, eddy current loss should be lowered by high resistivity of the magnet. Examples of preferable inorganic insulating films are disclosed in Japanese patent laid-open 10-154613 and are films formed by using a phosphate salt treating solution containing phosphoric acid, boric acid, and magnesium ions. In order to secure a constant thickness of the films and magnetic characteristics of the magnetic powder, a surfactant and an inhibitor should be added to the solution.

Particularly, perfluoroalkyl group surfactants and benzotriazole group inhibitors are preferable. However, these additives should not be used when the rare earth element magnetic powder is subjected to high temperature treatment, the magnetic powder is damaged.

An insulating film, which can be used together with the high temperature treatment, is fluoride coat film. This film improves not only insulation of the magnetic powder, but also improves magnetic characteristics of the magnet. As a solution for forming the fluoride coat, there are solutions wherein fluoride of rare earth elements or alkaline earth metals is swollen in alcohol solvent. The fluoride having a particle size of 10 μm or less is dispersed in the alcohol solvent in a sol state.

In order to improve magnetic characteristics of the magnets, the magnetic powder treated with the fluoride coat treating solution should be heat treated at a temperature higher than the conventional bond magnets but lower than sintering temperature. 

1. A rare earth element magnet comprising molded magnetic powder containing at least one rare earth element, wherein a Fe rich phase covering a part or entire of the surface of particles of the magnetic powder and having a Fe atomic percentage larger than that of the magnetic powder, and an inorganic binder bonding the particles covered with the Fe Rich phase.
 2. The rare earth magnet according to claim 1, wherein the Fe rich phase contains at least one rare earth element and oxygen.
 3. The rare earth element magnet according to claim 1, wherein the magnetic powder has a main phase of Nd₂Fe₁₄B, and the Fe rich phase is an oxide containing Nd and Fe, an atomic ratio of Fe to Nd+Fe being 50% or more.
 4. The rare earth element magnet according to claim 1, wherein the Fe rich phase is at least one member selected from the group consisting of Ge₁₇Nd₂, Nd_(4.4)Fe_(77.8)B_(17.8), NdFe₃(BO₃)₄ and Fe.
 5. The rare earth element magnet according to claim 1, wherein the inorganic binder is SiO2 containing an alkoxy group.
 6. The rare earth element magnet according to claim 1, wherein a high resistance film having a thickness of 10 nm to 10 μm is formed between the surface of the particles of the magnetic powder and the inorganic binder.
 7. The rare earth element magnet according to claim 6, wherein the high resistance film comprises at least one oxyfluoride compound.
 8. The rare earth element magnet according to claim 7, wherein the oxyfluoride is a fluoride of rare earth element and/or fluoride of alkaline earth metal.
 9. A method of manufacturing rare earth element magnet comprising: compacting magnetic powder containing a rare earth element to form a molded magnet body; heating the molded magnet body to form a Fe rich phase having a Fe atomic percentage higher than that of a mother phase constituted by the magnetic powder; impregnating the molded magnet with a solution of a precursor of an inorganic oxide binder; and Heating the molded magnet impregnated with the precursor solution to cure the precursor so as to form an inorganic binder.
 10. A method of manufacturing rare earth element magnet comprising: adding a treating solution containing a fluoride to magnetic powder containing rare earth element; forming a fluoride coat film on the surface of the particles of the magnetic powder; compacting the magnetic powder to form a molded magnet body; heating the molded magnet body to form a Fe rich phase having a Fe atomic percentage higher than that of a mother phase constituted by the magnetic powder; impregnating the molded magnet with a solution of a precursor of an inorganic oxide binder; and heating the molded magnet impregnated with the precursor solution to cure the precursor so as to form an inorganic binder.
 11. The method of manufacturing rare earth element according to claim 10, wherein the treating solution containing a sol state fluoride contains swollen fluoride of rare earth element and/or alkaline earth element suspended in solvent whose main component is alcohol.
 12. The method of manufacturing rare earth element according to claim 10, wherein the fluoride of rare earth element and/or alkaline earth element has a particle size of 10 μm or less.
 13. The method of manufacturing rare earth element according to claim 10, wherein the alcohol is at least one of methyl alcohol, ethyl alcohol, n-propylalcohol and isopropyl alcohol. 